With the spread of power supplies for driving motors of portable electronic devices, electric automobiles, and so on, research and development of lithium-ion secondary batteries as a kind of non-aqueous electrolyte secondary battery that can achieve superior battery characteristics such as high-energy density and high output is being carried out. As cathode active material that is used as the cathode material for a lithium-ion secondary battery, there is lithium metal composite oxide having a layered or spinel structure. Lithium-ion secondary batteries that use this lithium metal composite oxide can obtain a 4V class high voltage, and have high-energy density, so implementation is advancing.
As a lithium metal composite oxide, there is currently lithium-cobalt composite oxide (LiCoO2) that can be comparatively easily synthesized, lithium-nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium-nickel-cobalt-manganese composite oxide (LiNi1/3CO1/3Mn1/3O2), lithium-manganese composite oxide (LiMn2O4) that uses manganese, lithium-nickel-manganese composite oxide (LiNi0.5Mn0.5O2) and the like. Of these, lithium-nickel-manganese composite oxide has a layered structure that is the same as lithium-cobalt composite oxide or lithium-nickel composite oxide, and normally, includes nickel and manganese in transition metal sites at a ratio of 1:1, does not use cobalt of which there are small deposits, and is capable of achieving superior battery characteristics such as excellent thermal stability and high capacity, so it gaining attention as a cathode active material (refer to “Chemistry Letters, Vol. 30 (2001), No. 8”, pg. 744).
In order for secondary batteries that use lithium-nickel-manganese composite oxide as cathode active material to have excellent battery characteristics such as a high cycling characteristic, low resistance, and high output, preferably the cathode active material has a comparatively small particle size, narrow particle-size distribution, and high filling characteristic. In order to accomplish this, together with improving the morphology (shape and form) of the cathode active material and precursor of that cathode active material, and bringing the shape of the cathode active material closer to a spherical shape, controlling the average particle size and particle-size distribution so as to be within a suitable range is being studied. Morphology means characteristics related to the shape, form and structure of particles such as the outer shape, average particle size, the extent of particle-size distribution, primary particles, tap density and the like.
Here, as methods for producing lithium-nickel-manganese composite oxide, there are methods such as a method of wet mixing nickel oxide powder, manganese oxide powder and lithium oxide powder, after which the mixture is sprayed and dried to form a granulated powder and then calcining that powder, and a method of precipitating out nickel-manganese composite hydroxide by a crystallization reaction, then mixing that nickel-manganese composite hydroxide with a lithium compound and calcining the mixture. Of these methods, the method of using a crystallization reaction is such that by suitably regulating the reaction conditions, not only is it possible to uniformly disperse nickel and manganese on a molecular level, it is also possible to obtain a nickel-manganese composite hydroxide having a suitable particle size, a narrow particle-size distribution, and excellent filling characteristic. Moreover, generally, the lithium-nickel-manganese composite oxide takes on the particle characteristics of the precursor, so by using this kind of nickel-manganese composite hydroxide as a precursor, it is possible to obtain lithium-nickel-manganese composite oxide that not only has a uniform composition, but also has a suitable particle size, narrow particle-size distribution and excellent filling characteristic.
JP2006-252865 (A), JP2008-266136 (A) and WO2004/092073 (A1) disclose methods of crystallizing nickel-cobalt-manganese composite hydroxide by continuously or intermittently supplying a nickel-cobalt-manganese salt aqueous solution, an alkali aqueous solution, and an aqueous solution that includes an ammonium-ion donor into a reaction tank. However, in the methods disclosed in that literature, the morphology of the obtained nickel-cobalt-manganese composite hydroxide is low, and it is not possible to sufficiently improve the filling characteristic of the cathode active material produced using that nickel-cobalt-manganese composite hydroxide as a precursor.
On the other hand, WO2013/125703 (A1) discloses a production method for producing nickel composite hydroxide in which a nickel composite hydroxide slurry is obtained while performing control so that the ratio of the average particle size per volume of secondary particles of nickel composite hydroxide that is generated inside a reaction vessel with respect to the average particle size per volume of secondary particles of the finally obtained nickel composite hydroxide is 0.2 to 0.6, after which, only the liquid component of the slurry is continuously removed with the amount of slurry being kept constant, and the crystallization reaction is continued until the size of the secondary particles becomes fixed. WO2013/125703 (A1) discloses that the nickel amine complex concentration in the reaction aqueous solution is preferably regulated to be within the range of 10 mg/L to 1,500 mg/L. With this kind of production method, it is considered possible to obtain a nickel composite hydroxide having a moderately large particle size and narrow particle-size distribution. However, the nickel composite oxide that is disclosed in WO2013/125703 (A1) is produced mainly for the purpose of improving the uniformity of particle size. Therefore, even though the shape of the obtained composite hydroxide particles is nearly spherical, the particles having a ratio of the minimum diameter and maximum diameter (minimum diameter/maximum diameter) in outward appearance to be at about 0.6 are allowed. Particularly, in the case of a nickel composite hydroxide that includes manganese, a considerable amount of particles having an elliptical shape are also included, so it is difficult to say that this nickel composite hydroxide has high morphology, and it is not possible to improve the filling characteristic of the cathode active material which is produced using this nickel composite hydroxide as a precursor.
Here, as an index for evaluating the morphology of spherical powder, there is the sphericity. However, in the case of production on an industrial scale, performing evaluation by measuring the cathode active material or nickel-manganese composite hydroxide in three dimensions and calculating the sphericity is not practical. Therefore, the morphology of the powder is typically evaluated more simply by measuring the cathode active material or nickel-manganese composite hydroxide in two dimensions, and calculating the roundness.
For example, JP2008-077990 (A) and JP2009-076388 (A) disclose a cathode active material that includes secondary particles of lithium-nickel-cobalt composite oxide that is represented by the formula: LixNi1-y-zCoyMezO2 (where 0.85≤x≤1.25, 0<y≤0.5, 0≤z≤0.5, 0<y+z≤0.75, and the element Me is at least one element selected from among Al, Mn, Ti, Mg and Ca), and it is described that preferably the average value of the roundness of an arbitrary 100 particles that have an equivalent circle diameter that matches the average particle size becomes 0.88 or greater. According to JP2008-077990 (A), this kind of cathode active material has higher thermal stability than cathode active material that includes secondary particles having an indeterminate lump-like shape. Moreover, according to JP2009-076383 (A), by aggressively mixing the aqueous solution inside the reaction tank when generating the precursor nickel-cobalt composite hydroxide by a crystallization reaction, it is possible to make cathode active material having a roundness of 0.88 or greater.